1. Technical Field
The invention relates to permeable base transistors.
2. Discussion of Related Art
Permeable base transistors (PBTs) offer advantages in speed and packing density over conventional field effect transistors (FETs) (Wemersson et al., Mat. Sci. and Ens. B 51:76-80 (1998); Nilsson et al., Solid State Elec. 42:297-305 (1998)). In typical PBT technologies, a metallic base layer is overlaid onto a single crystal semiconductor substrate (emitter/collector) to form a Schottky barrier (e.g., U.S. Pat. No. 4,378,629). A second epitaxial semiconductor layer is overgrown on the base layer (second collector/emitter). The base layer is patterned with openings so that current can flow from emitter to collector only when a voltage is applied to the base layer. A variety of metals, such as tungsten and metal silicides (e.g., WSi2, NiSi2 and CoSi2) have been used as materials for the base layer (von Kxc3xa4nel, Mat. Sci. Rep. 8:193-269 (1992); Zaring et al., Rep. Progress Phys. 56:1397-1467 (1993); Pisch et al., J. App. Phys. 80:2742-2748 (1996)). These PBTs were predicted to have high gains at very high frequencies (200 GHz), which were not achievable with conventional FET technologies. However, problems such as poisoning of PBT semiconductor structures by metal electromigration, insufficient heat dissipation, and complexity of epitaxial overgrowth to form embedded metal base layers have prevented mass fabrication of PBTs (Hsu et al., J. App. Phys. 69:4282-4285; Miyao et al., J. Cryst. Growth 111:957-960 (1991)).
Therefore, a need exists for new PBTs that provide the predicted improvements in speed, packing density, and high frequency performance over FETs, without suffering from the drawbacks associated with the metal base layers of traditional PBTs.
The invention provides a method of making a permeable base transistor (PBT) having a base layer that includes nanotubes. One aspect of the invention provides a method of making a permeable base transistor. According to the method, a semiconductor substrate is provided, a base layer is provided on the substrate, and a semiconductor layer is grown over the base layer. The base layer includes metallic nanotubes.
In some embodiments, the base layer is formed by growing a carbon nanotube fabric on the substrate using a catalyst. In certain embodiments, the catalyst is a gas-phase catalyst. In particular embodiments, the catalyst is a metallic gas-phase catalyst. In other embodiments, the base layer is formed by depositing a solution or suspension of nanotubes on the substrate. In certain embodiments, the solution or suspension is deposited by spin-coating. In particular embodiments, the solution or suspension is deposited by dipping the substrate into the solution or suspension. In still other embodiments, the base layer is formed by spraying an aerosol having nanotubes onto a surface of the substrate.
In some embodiments of the method, the base layer is patterned. In certain embodiments, an ohmic contact is provided in communication with the substrate. In particular embodiments, an ohmic contact is provided in communication with the semiconductor layer. In particular embodiments, the nanotubes include single-walled carbon nanotubes. In certain embodiments, the base layer includes a monolayer of carbon nanotubes.